1. Field of the Invention
The present invention relates to filters for alternating current circuits, and more particularly relates to passive filters to remove undesirable harmonics generated on a single-phase alternating current power line servicing non-linear loads.
2. Description of Related Art
The IEEE "Recommended Practices and Requirements for Harmonic Control and Electrical Power Systems", IEEE standard 519-1992; IEEE press April 1993, indicates that harmonic currents generated by nonlinear loads create problems in an industrial/commercial multi-user environment. Single-phase AC to DC rectifier circuits such as the rectifier circuits including diodes D1-D4 illustrated in prior art drawing, FIG. 1, which are connected to DC filter capacitors C as part of and in parallel with the load R, draw pulsating currents from the alternating current source (AC source). The shape of the current pulses is shown in FIG. 2A which indicates that pulses are 180 degrees apart. Using Fourier analysis, it can be shown that such wave forms are rich in "triplen" harmonics, i.e. wave forms having frequency components which are odd multiples of three (3rd, 9th, 15th, etc.). The current wave form is also rich in the other odd order harmonics (for example, the 5th and the 7th harmonics). The magnitude of the pulse currents and hence the magnitude of the harmonics is a function of the load R connected across the DC filter capacitor C.
Some typical problems associated with current harmonics generated by nonlinear loads are: (1) increased heating in transformers supplying the nonlinear load; (2) the requirement for increased "ampacity" (capability of conductors to carry current) of conductors; (3) radio and telephone interference, and; (4) interference with other electronic controllers. Moreover, an alternating current source charging a DC filter capacitor without intervening impedance or low impedance generates a flat-topped or distorted AC voltage wave form. Flat topping or clipping is a principal cause for voltage harmonics at the input of the load and can translate to network resonance problems farther back along the line.
In addition to the aforementioned problems created by nonlinear or distorted wave forms is that the power supplied to nonlinear loads, such as to a personal computer, fax machines, telephones, data communication equipment and the like, may be violative or could lead to a violation of the recently commissioned European standards and the IEEE 519 1992 harmonic recommendations. Single-phase harmonic filters will therefore soon be essential to circuits with single-phase, nonlinear loads.
It is accepted in the art that the harmonic content in a given wave form is best depicted by the term "Total Harmonic Distortion" (THD). THD is used to define the effective harmonics on the power system. THD is defined as the ratio of the root-mean-square of the harmonic content to the root-mean-square value of the fundamental quantity (i.e., in a AC power line, 50/60 Hz component) expressed in percent. Mathematically this is in equation 1 below. ##EQU1##
Shunt tuned harmonic filters such as taught in U.S. Pat. No. 4,930,061 issued on May 29, 1990 to Slack, et al., are commonly used for trapping harmonics generated by diode rectifier circuits with DC filter capacitors such as shown in FIG. 1. These single-phase harmonic filters are generally tuned to a cut-off frequency approaching the third harmonic. This type of filter, such as shown in FIG. 3 of U.S. Pat. No. 4,930,061 offers only capacitive impedance at the fundamental frequency. This capacitive impedance creates an overvoltage across the line terminals which affects other equipment connected across the same line. A further disadvantage of shunt tuned harmonic filters is that they tend to import harmonics from other nonlinear loads connected to the same network causing overloading of the filter. In the circuit of FIG. 1, in addition to the third harmonic components, a bridge rectifier also generates higher order odd harmonics (5th, 7th) that require the addition of extra tuned branches in the shunt tuned harmonic filter to filter out the higher order harmonics. Such a shunt tuned harmonic filter employing 7th harmonic, 5th harmonic and high pass sections is illustrated in U.S. Pat. No. 3,555,291 issued on Jan. 12, 1971 to Dewey.
Shunt tuned harmonic filters, moreover, are also notorious for setting up harmonic resonance within a power system if improperly designed.
In order to overcome detuning of shunt tuned harmonic filters and also to remove residual ripples which the tuned filters are unable to remove, hybrid filtering techniques have been employed. In one such scheme discussed in U.S. Pat. No. 3,849,677, issued on Nov. 19, 1974 to Stacey et al., a shunt active path is provided. While this technique may be effective, it is cumbersome and requires additional components and thus increased expense because each shunt harmonic filter has to have at least one shunt active portion.
An entirely active approach has also been employed and is illustrated in the prior art in U.S. Pat. No. 4,812,669 issued on Mar. 14, 1989. While such techniques are moderately successful, the systems still are complicated, have a higher component or part count and require passive filters to filter out higher order harmonic currents.
In U.S. Pat. No. 5,444,609 issued on Aug. 22, 1995 to the present inventor (Swamy et al.), the harmonic filter disclosed therein includes series inductance, a parallel capacitance and a transformer. The filter disclosed therein is specifically adapted for variable frequency drives and is not applicable to single-phase nonlinear loads as presented in the present application. Moreover, there exists no series blocking tuned filter, as in the present application.
Another approach to reducing harmonic content in voltage and current wave form sources has been the "boost converter techniques" which shape the wave on the input AC current. This has been well documented in "An Active Power Factor Correction Technique for Three-Phase Diode Rectifiers" by A. R. Prasad, P. D. Ziogas, and S. Manias, IEEE Transactions on Power Electronics, January 1991, Volume 6, No. 1, pages 83-92; and in "Analysis and Design of a New Three-Phase Power Conditioner Providing Sinusoidal Input Currents and Multiple Isolated DC Outputs", by Johann W. Kolar, Hans Ertl, and Franz C. Zach as documented in the Proceedings of the 26th International Conference on Power Conversion, Nurnberg, Germany, June 1993, pages 151-165. However, with boost converter techniques, the switching stress is high and there is in general about a 40% higher voltage on the DC side than is normally acceptable. Thus, boost type AC to DC converters are not as popular with rectifier type loads. However, arc discharge and fluorescent lamps require higher voltages to trigger and hence such technology is acceptable with such loads in order to mitigate the effects of harmonics.